Field of the Invention
The present invention relates to a fuel cell system and a fuel cell system control method.
Description of Related Art
As a fuel cell, there is known a fuel cell stack, in which: a membrane electrode structure is formed by sandwiching a solid polymer electrolyte membrane from both sides thereof with an anode electrode and a cathode electrode; a pair of separators are arranged on both sides of this membrane electrode structure to configure a flat plate-shaped unit fuel cell (hereunder, referred to as “unit cell”); and a plurality of these unit cells are laminated. In this fuel cell, hydrogen gas serving as a fuel gas is supplied to an anode gas flow path that is formed between the anode electrode and the anode side separator, and air serving as an oxidant gas is supplied to a cathode gas flow path that is formed between the cathode electrode and the cathode side separator. As a result, hydrogen ions generated at the anode electrode as a result of a catalytic reaction permeate through the solid polymer electrolyte membrane and move to the cathode electrode, and at the cathode electrode, they electrochemically react with oxygen within the air, thereby causing electric power generation to be performed.
A fuel cell system is provided with a cooling device for cooling a fuel cell that generates heat as electricity is generated. For example, the fuel cell system disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-67394 is mounted on a vehicle, and is provided with a radiator side flow path that supplies coolant to a radiator, a bypass flow path that bypasses the coolant that has cooled a fuel cell stack from the radiator, and a thermostat valve that makes the flow rate of the coolant flowing into the radiator side flow path greater when the coolant temperature is high, compared to that when the coolant temperature is low. The switching temperature of the thermostat valve is set to a fixed value on the basis of low altitude travelling.
The thermostat valve shuts off the bypass flow path and allows coolant to flow only into the radiator side flow path when the temperature of coolant becomes higher than the switching temperature. On the other hand, it shuts off the radiator side flow path and allows coolant to flow only into the bypass flow path when the temperature of coolant becomes lower than the switching temperature.
Furthermore, the fuel cell system disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-67394 is provided with an electric heater that heats coolant, and a control device that controls the temperature of the coolant. The control device controls the electric heater based on coolant temperature and outside air temperature, and controls the temperature of coolant flowing into the fuel cell according to the outside air temperature.
For example, at a high altitude location where air pressure is low, the electric heater is operated to heat coolant that flows into the thermostat valve, and the thermostat valve is forced to operate to switch the coolant path to the radiator side flow path, to thereby perform coolant temperature control. Moreover, at a low altitude location where air pressure is high, the thermostat valve, in which the switching temperature is set on the basis of low altitude travelling, automatically switches between the radiator side path for allowing coolant to flow through the radiator, and the bypass flow path for bypassing the radiator, to thereby perform coolant temperature control.
According to Japanese Unexamined Patent Application, First Publication No. 2010-67394, it is claimed that: it is possible, at a high altitude location where air pressure is low, to prevent the membrane from drying out by controlling the temperature of coolant to be supplied to the fuel cell to a lower temperature; and it is possible, at a low altitude location where air pressure is high, to prevent excessive moisture by controlling the temperature of coolant to be supplied to the fuel cell to a higher temperature, and therefore, the wet state of the membrane of the fuel cell can always be maintained within an appropriate range regardless of the travelling environment of the vehicle.
Incidentally, in a fuel cell system, in some cases, oxygen and hydrogen may remain in the interior of the fuel cell after electric power generation is stopped. These remaining oxygen and hydrogen are known to oxidize the anode electrode and the cathode electrode and cause the fuel cell to deteriorate. Consequently, rapid cooling of a fuel cell after electric power generation is desired in order to suppress oxidation of the anode electrode and the cathode electrode.